Topic Contents

Prothrombin Time and INR

Test Overview

Prothrombin time (PT) is a blood test that measures how
long it takes blood to clot. A prothrombin time test can be used to check for
bleeding problems. PT is also used to check whether medicine to prevent blood
clots is working.

A PT test may also be called an INR test. INR
(international normalized ratio) stands for a way of standardizing the results
of prothrombin time tests, no matter the testing method. So your doctor can
understand results in the same way even when they come from different labs and
different test methods. In some labs, only the INR
is reported and the PT is not reported.

Blood
clotting factors are needed for blood to clot
(coagulation). Prothrombin, or factor II, is one of the clotting factors made
by the liver. Vitamin K is needed to make prothrombin and other clotting
factors. Prothrombin time is an important test because it checks to see if five
different blood clotting factors (factors I, II, V, VII, and X) are present.
The prothrombin time is made longer by:

Blood-thinning medicine, such as warfarin.

Low levels of blood clotting factors.

A change in
the activity of any of the clotting factors.

The absence of any of
the clotting factors.

Other substances, called inhibitors, that
affect the clotting factors.

An increase in the use of the clotting factors.

An abnormal prothrombin time is often caused by liver
disease or injury or by treatment with blood thinners.

Another
blood clotting test, called partial thromboplastin time (PTT), might be used if you take another type of blood-thinning medicine called heparin. This test measures other
clotting factors. Partial thromboplastin time and prothrombin time are often
done at the same time to check for bleeding problems or the chance for too much
bleeding in surgery.

Why It Is Done

Prothrombin time (PT) is measured
to:

Find a cause for abnormal bleeding or
bruising.

Check the effects of warfarin (Coumadin). You will have the test regularly to make sure you are taking the right dose.

Check for low levels of blood clotting factors.
The lack of some clotting factors can cause bleeding disorders such as
hemophilia, which is passed in families
(inherited).

Check for a low level of vitamin K. Vitamin K is
needed to make prothrombin and other clotting factors.

Check if it is safe to do a procedure or surgery that might cause bleeding.

Check how
well the liver is working. Prothrombin levels are checked along with other
liver tests, such as aspartate aminotransferase and alanine
aminotransferase.

How To Prepare

Many medicines can change the results
of this test. Be sure to tell your doctor about all the nonprescription and
prescription medicines you take, as well as any supplements or herbal remedies
you use.

How It Is Done

The health professional drawing blood
will:

Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.

Clean the needle site with
alcohol.

Put the needle into the vein. More than one needle stick
may be needed.

Attach a tube to the needle to fill it with
blood.

Remove the band from your arm when enough blood is
collected.

Put a gauze pad or cotton ball over the needle site as
the needle is removed.

Put pressure on the site and then put on a
bandage.

In some cases the health professional will take a sample of blood from your fingertip instead of your vein. For a finger stick blood test, the health professional will clean your hand, use a lancet to puncture the skin, and place a small tube on the puncture site in order to collect your blood.

How It Feels

The blood sample is taken from a vein in
your arm. An elastic band is wrapped around your upper arm. It may feel tight.
You may feel nothing at all from the needle, or you may feel a quick sting or
pinch.

Risks

There is very little chance of a problem from
having a blood sample taken from a vein.

You may get a small bruise at the site. You
can lower the chance of bruising by keeping pressure on the site for several
minutes.

In rare cases, the vein may become swollen after the
blood sample is taken. This problem is called phlebitis. A warm compress can be
used several times a day to treat this.

Ongoing bleeding can be a
problem for people with bleeding disorders. Aspirin, warfarin (such as Coumadin), and
other blood-thinning medicines can make bleeding more likely. If you have
bleeding or clotting problems, or if you take blood-thinning medicine, tell
your doctor before your blood sample is taken.

Results

Prothrombin time (PT) is a blood test
that measures how long it takes blood to clot.

Normal

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

A method of standardizing prothrombin time results, called
the international normalized ratio (INR) system, has been developed so the
results among labs using different test methods can be understood in the same
way. Using the INR system, treatment with warfarin (Coumadin) will be the same. In some labs, only the INR is
reported and the PT is not reported.

The warfarin (Coumadin) dose is changed so that the
prothrombin time is longer than normal (by about 1.5 to 2.5 times the normal
value or INR values 2 to 3). Prothrombin times are also kept at longer times
for people with artificial heart valves, because these valves have a high
chance of causing clots to form.

Abnormal values

A longer-than-normal PT can mean a lack of or
low level of one or more blood clotting factors (factors I, II, V, VII, or X).
It can also mean a lack of vitamin K; liver disease, such as
cirrhosis; or that a liver injury has occurred. A
longer-than-normal PT can also mean that you have
disseminated intravascular coagulation (DIC), a
life-threatening condition in which your body uses up its clotting factors so
quickly that the blood cannot clot and bleeding does not stop.

A
longer-than-normal PT can be caused by treatment with blood-thinning medicines,
such as warfarin (Coumadin) or, in rare cases, heparin.

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

Taking medicines that can affect the action of
blood thinners (such as warfarin) and vitamin K. These include antibiotics,
aspirin, cimetidine (Tagamet), barbiturates, birth control pills, hormone therapy (HT), and vitamin K
supplements.

Having severe diarrhea or vomiting that causes fluid
loss and dehydration. This may make the PT time longer. If diarrhea is caused
by poor absorption of nutrients, vitamins, and minerals from the intestinal
tract (malabsorption syndrome), the PT may be longer because of a lack of
vitamin K.

Eating foods that have vitamin K, such as broccoli, chickpeas, kale, turnip greens, and soybean
products.

Drinking a lot of alcohol.

Taking some herbal products or natural remedies.

What To Think About

The U.S. Food and Drug Administration (FDA) has
approved a home test for prothrombin time (PT). If you need a PT test frequently and for a long time, you may want to ask your doctor if this home test is an option for you.

A PT is done at the same time of day
each time so test results can check whether the right dose of warfarin is being
used to prevent blood clots.

Another blood clotting test, called
partial thromboplastin time (PTT), measures other clotting factors. Partial
thromboplastin time and prothrombin time are often done at the same time to
check for bleeding problems. To learn more, see the topic
Partial Thromboplastin Time.

Prothrombin
levels are checked along with other liver tests, such as aspartate
aminotransferase and alanine aminotransferase to check how the liver is
working.

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